We have previously shown in our lab that responses of peripheral blood mononuclear cells (PBMC) to whole heat-killed Borrelia burgdorferi spirochetes (h-Bb) are down-regulated in tick-inoculated rhesus monkeys with chronic infections and also in a cohort of animals with acute infections. In addition, blastogenesis of PBMC in response to recombinant (nonlipidated) p39 and p35 and to lipidated OspA were performed in these animals; no responses were observed. More recently, we assessed blastogenesis of PBMC and of brachial lymph nodes from 8 needle-inoculated rhesus monkeys (3 inoculated with JD1 spirochetes; 5 inoculated with a neurotropic strain NT1), in response to h-Bb (JD1 and NT1); their cells also failed to respond to antigens. However, in spite of the PBMC unresponsiveness to antigens, Con A responses were consistently high in all animals suggesting, a specific modulation of the B. burgdorferi antigenic responses in these animals. Given the inability of PBMC from tick and needle-inoculated animals to significantly respond to JD1 or NT1 h-Bb we decided to focus more on the mechanisms that might contribute to the observed unresponsiveness phenomenon. Therefore, RNA from PBMC that had or had not been exposed to JD1 or NT1 h-Bb or to Con A were collected to determine the role that cytokines might play in modulating responses to antigen in inoculated animals. The RNA was used to assess, by reverse transcriptase polymerase chain reaction (RT-PCR), the expression of IL-2, IL-4, IL-10 and IFN- cytokine genes. Compared to unstimulated cells, there was a marked upregulation of the expression of the IL-10 gene, with a corresponding down-regulation of the IL-2 gene expression in cells that were stimulated with JD1 h-Bb in all of the tick inoculated and control animals. In contrast, an upregulation of the expression of the IL-10 and IL-2 cytokine genes in [unreadable]cells stimulated with either JD1 or NT1 h-Bb was seen in all needle inoculated animals. There was no difference in the expression of IL-4 and IFN- genes between unstimulated and antigen-stimulated cells. The studies show that h-Bb cause an upregulation of the expression of the IL-10 gene. The enhanced transcription of the IL-10 gene that we have observed suggests that IL-10 could be abundantly produced and secreted in vitro in response to h-Bb. The production of other cytokines could be down-regulated or abrogated as a consequence. This hypothesis will be tested with the aid of a commercially-available anti-human IL-10 MAb. If this antibody is able to functionally impair rhesus IL-10 then non-responsive PBMC from B. burgdorferi-infected animals should respond to h-Bb in the presence of the anti-IL-10 MAb. Finally, our observation may not only explain our in vitro results but may also be relevant in vivo.